I. Field of the Invention
This invention relates generally to an electronic control circuit for controlling the operation of a motor vehicle's windshield wiper blades, and more particularly to an electronic control circuit responsive to the presence of rain to automatically activate the vehicle's windshield wipers at a rate dictated by the level of precipitation being encountered.
II. Discussion of the Prior Art
Heretofore, motor vehicles having windshields have been equipped with motor-driven windshield wipers and associated with the wiper motor is a manually-operated multiposition switch whereby the driver can turn on and select the speed at which the wiper blades will be driven. While earlier model vehicles generally allowed selection of only a slow and a fast wiper sweep speed, more recently, wiper controls have also included a delay circuit whereby the operator can manually adjust the duty cycle of the wiper motor.
The present invention comprises an improvement over the aforementioned manually-operated windshield wiper motor controls which permits the automatic operation of the windshield wiper motor when rain droplets collect on the surface of the windshield. Broadly speaking, other such automatic control circuits are known in the art. For example, in the Fukatsu et al U.S. Pat. No. 4,476,419, there is described a windshield wiper control device which includes a light source mounted on the vehicle's hood outside of the passenger compartment which directs a beam of light onto a sensor attached to the vehicle's rearview mirror located within the passenger compartment. The presence of rain on the windshield disturbs the pattern of light transmitted through the windshield, causing a circuit response to the change in contrast to develop a signal for the wiper motor to cause it to turn on and to sweep at a rate determined by the level of precipitation. PG,3
In the Noack U.S. Pat. No. 4,355,271, a plurality of infrared light sources are arranged to transmit IR light from the interior of the windshield toward its exterior surface and a common IR sensor is positioned to normally intercept the IR energy reflected back from the air-glass interface at the outside surface of the glass. The presence of rain or dust particles on the exterior windshield surface affects the amount of IR reflected back on to the sensor and the resulting signal shift is compared in a series of comparators with predetermined threshold values whereby a determination is made whether water droplets or dust particles are present on the windshield. If it is dust-particles, the windshield washer pump is turned on along with the wiper blades to clear the windshield. On the other hand, if it is water droplets that is causing the shift in the IR energy reaching the sensor, then only the wiper blades are activated.
Other windshield wiper control schemes are described in the Blaszkowski U.S. Pat. No. 4,131,834; the Gibson U.S. Pat. No. 4,463,294; and the Boegh-Peterson U.S. Pat. No. 4,495,452. A system for automatically operating the window motors to close the windows of a vehicle upon the detection of rain is set out in the Watanabe et al U.S. Pat. No. 4,481,450.
The present invention is deemed to offer significant advantages over the windshield wiper control systems of the prior art discussed above. As will be described in greater detail hereinbelow, the control circuitry of the present invention carries out its desired function in a highly reliable manner over prolonged periods of use and under a wide variety of adverse ambient conditions.